The present invention relates to a charge/discharge control circuit which can control the charge/discharge of a secondary cell and further relates to a chargeable electric power source apparatus using the charge/discharge control circuit.
A conventional electric power source apparatus provided with a secondary cell or battery is shown in a block diagram of FIG. 2. For example, such a type of apparatus is disclosed in Japanese Patent Application Laid-Open No. Hei 4-75430 entitled "Chargeable Type Electric Power Source Apparatus". More specifically, a secondary cell 101 is connected to an external terminal -V.sub.0 or +V.sub.0 through a switching circuit 103. Further, A charge/discharge control circuit 102 is connected in parallel to the secondary cell 101. The charge/discharge control circuit 102 has a function of detecting a voltage of the secondary cell 101. In either an overcharged condition (where the voltage is kept higher than a predetermined value) of the secondary cell 101 or an overdischarged condition (where the voltage is kept lower than a predetermined voltage value), a signal for turning off the switching circuit 103 is fed from the charge/discharge control circuit 102. Accordingly, in the overcharged condition, the switching circuit 103 is turned off, so that a charge is interrupted to the secondary cell 101 from a primary electric power source which is connected to the external terminals -V.sub.0 and +V.sub.0. In case of the overdischarged condition, the switching circuit 103 is turned off, so that a supply of the energy to a load (for example, a portable telephone using the secondary cell) is interrupted. Namely, the charge/discharge control circuit 102 controls the switching circuit 103 between the secondary cell 101 and the external terminals, thereby preventing a charge larger than necessary from the external terminals to the secondary cell 101 and at the same time preventing an excessive chargeability degradation of the secondary cell caused by an energy supply from the secondary cell 101 to the load connected to the external terminals.
Also, a chargeable electric power source apparatus as shown in a block diagram of FIG. 30 is known as another conventional example. In FIG. 30, a secondary cell 101 is connected to an external terminal -V.sub.0 or +V.sub.0 through a switching circuit 103 and a current sensing resistor 104. Further, A charge/discharge control circuit 102 is connected in parallel to the secondary cell 101 and an overcurrent detecting circuit 105. The charge/discharge control circuit 102 has a function of detecting a voltage of the secondary cell 101. In either an overcharged condition of the secondary cell 101 or an overdischarged condition, a signal for turning off the switching circuit 103 is output from the charge/discharge control circuit 102. Also, in the case where an accident occurs in the load to cause the overcurrent condition, a comparator 21 monitors the voltage of the current sensing resistor 104 and compares it with a voltage of a reference voltage circuit 106.
It is assumed that V.sub.REF [V] is the voltage value of the reference voltage circuit 106, R[.OMEGA.] is the resistance value of the current sensing resistor 104 (in this case, the ON-resistance of the switching circuit 103 is assumed to be much smaller than R), and I [A] is the current flowing in this case, the current is represented as following equation (1). EQU I=V.sub.REF /R [A] (1)
In this case, the output of the comparator 21 is changed from "H" to "L", a transistor 107 is turned off, a capacitor 109 is charged by a constant current source 108, and after a certain time lag, the output of a comparator 302 is changed from "H" to "L", and the switching circuit 103 is turned off. Namely, the constant current source 108, the capacitor 109 and the transistor 107 constitute a time delay circuit for delaying the output of the comparator 302. The delayed signal is input into the comparator 302 together with the signal of the reference voltage circuit 106. These signals are compared with each other in the comparator 302. The output turns off the switching circuit 103.
Another conventional electric power source apparatus provided with a secondary cell and a charge/discharge control circuit is shown in a block diagram of FIG. 37. For example, such a type of apparatus is disclosed in Japanese Patent Application Laid-Open No. Hei 4-75430 entitled "Chargeable Type Electric Power Source Apparatus". More specifically, a secondary cell 24 and a charge/discharge controlling IC 374 are connected to an external terminals +V and -V through switching transistors 372 and 373, respectively.
For instance, in the case where the voltage of the secondary cell 24 exceeds an overcharged voltage when a charging electric source is connected to the external terminals +V and -V, the switching transistor 372 is switched over from the "ON" condition to "OFF" condition to thereby stop the charge from the external terminals to the secondary cell 24. Conversely, in the case where a portable equipment such as a video camera or the like is connected to the external terminals and electric charge is supplied from the secondary cell 24 to the portable equipment, if the voltage of the secondary cell drops below the overdischarged voltage, the switching transistor 373 is switched over from the "ON" condition to "OFF" condition to thereby stop the discharge. One of the transistors 372 and 373 functions as a "transistor" and the other functions as a "diode". The functions as the transistor and the diode are alternatively used in accordance with the charging or discharging condition. A substrate of each transistor is connected to an associated source so as to be capable of functioning as the diode.
The conventional charge/discharge control circuit as shown in FIG. 2, has a drawback of a large power consumption to shorten the service life of the secondary cell of the energy supply source. As a result, the period of time of use of the equipment to be driven by the secondary cell would be shortened. Further, under the overdischarged condition were the chargeability capacity of the secondary cell is degraded, even though the energy supply from the secondary cell to the external equipment by the switching circuit is interrupted, the power consumption of the charge/discharge control circuit per se provided within the power source apparatus causes the charge to be further discharged to accelerate the degradation of the cell and the service life thereof.
In order to overcome the above-noted short-comings inherent in the prior art system, an object of the present invention is to provide a chargeable power source apparatus with a long service life secondary cell by reducing the power consumption of the charge/discharge control circuit.
The conventional charge/discharge control circuit shown in FIG. 30 suffers from the following defects. Namely, under the condition that a charger is connected with the terminals -V.sub.0 and +V.sub.0 from the outside and the secondary cell 101 is charged, when the secondary cell is fully charged, the switching circuit 103 is turned off. By this "OFF" operation, the potential between both ends of the secondary cell 101 is reduced, and the system is again kept under the charged condition, i.e., the switching circuit 103 is turned on. At the voltage after the completion of the charging operation, the detection of the full charge would be oscillated unstably.
As described in the description of the conventional system, when the overcharged condition appears during the charging operation to the secondary cell, the charge/discharge control circuit operates to turn off the switching circuit for controlling the charge to the secondary cell. However, since the charge/discharge control circuit is connected in parallel with the secondary cell, the current to be consumed during the operation is supplied from the secondary cell. The voltage drop occurs in the secondary cell by its supply of the current, and the voltage would be less than the overcharged detection voltage to thereby turn on the switching circuit. As a result, the operation is as follows: the voltage elevation of the secondary cell by the charging operation; the elevation up to the overcharged voltage; the voltage drop of the secondary cell by the operation of the charge/discharge control circuit; and the voltage elevation of the secondary cell by the charging operation again. The same operational steps are repeated so that the operation could not reach the overcharged condition. Also, in the case where the overdischarged condition is released during, the charging operation of the overdischarged cell, the conventional system suffers from the same problem.
Also, when the charge/discharge control circuit is for the first time connected to the secondary cell, unless a logic of the switching circuit is not secured, the initial condition would be unstable, and even if the voltage value of the secondary cell would be normal, the system would be brought into the overcharged condition or the overdischarged condition.
When the overdischarged condition of the secondary cell has developed so that the voltage value is less than a minimum voltage at which the voltage detection circuit within the charge/discharge control circuit or the control circuit is operated, the output of the voltage detection circuit or the control circuit would be unstable. Namely, since the voltage of the secondary cell is further decreased from the overdischarged condition, even if attempts are made to charge the secondary cell from the primary power source, the charge/discharge control circuit could not normally operate the switching circuit, as a result of which the charge is impossible. Namely, once the voltage of the secondary cell has been reduced less than the minimum voltage of the charge/discharge control circuit, since it is impossible to perform the charging operation, it is impossible to reuse the cell although it is a "secondary battery cell".
Another problem inherent in the conventional system will be explained. In the case where the charger is connected to both ends of the secondary cell and the secondary cell is charged, if the charge/discharge control circuit is connected under the condition that polarities of the charger are different from those of the secondary cell (i.e., so-called reverse connection), a CMOS IC which forms the charge/discharge control circuit is latched up so that the charge/discharge control circuit malfunctions to cause a large current to flow through the secondary cell to degrade the cell.
Still another problem in the conventional system will be explained. In the case where an accident occurs in a load which is connected to both ends of the secondary cell, if a large amount of current will flow from the secondary cell, the switching circuit 103 is turned off by the overcurrent detection circuit or the voltage of the secondary cell is rapidly increased by turning off this switching circuit so that the reference voltage value of the charge/discharge control circuit is increased. As a result, the switching circuit 103 is again closed to cause the oscillation.
In order to solve the problems inherent in the conventional system, an object of the present invention is to provide a charge/discharge control circuit which is not subject to the malfunctions described above.
Furthermore, in the case where two secondary cells are connected in series, the conventional system suffers from the following defects. Namely, the two secondary cells have respectively different consumption levels due to their service lives. However, if the sum of the voltages of the two cells may be kept over a certain level, there is no problem in use. In the conventional system, since each cell voltage is monitored, it is impossible to monitor the sum of the voltages. In some cases, even if it is possible to use the cells longer, the cells are no longer used. Thus, the period of time for use of the equipment would be shortened. Also, if the cell which suffers the local consumption is charged in the same manner as for the other normal cell, the local consumption is further accelerated to considerably reduce the service life of the cell.
Also, the chargeable electric power source apparatus shown in FIG. 37 suffers from the following defect. In this system, the two switching transistors are interposed between the external terminals and the secondary cell, and respective substrates are kept at potentials of the source electrodes of the transistor on the external terminal side and the transistor on the secondary cell side. Therefore, these transistors are assembled separately from the assembly of the charge/discharge control IC. As a result, it is difficult to make the battery compact and the assembly cost is high.
Accordingly, another object of the present invention is to provide a charge/discharge control circuit for a chargeable battery apparatus and a chargeable electric power source apparatus, which is small in size and low in cost with high reliability.